The invention relates to imaging devices, such as flat panel imagers. An imager is a device that receives electromagnetic radiation, e.g., light or x-rays, from the direction of something to be imaged in which an image is formed based upon the detected pattern of the radiation at the imager. A flat panel imager is a type of imager that comprises a matrix/array of detection elements, with each detection element providing a separate item of image data that is usable to reconstruct an image. For light-sensitive imagers, each detection element comprises a photosensitive device. For x-ray sensitive imagers, each detection element comprises an x-ray sensitive device.
FIG. 1 depicts one configuration of electrical components for a flat panel imager showing selectable wiring connections to transmit the voltage, current, or charge emitted by detection elements on the imager. Each image element 104 in the imager of FIG. 1 comprises a photodiode 106 that generates an electrical signal in response to a light input. A transistor 108 (such as a thin-film N-type FET) functions as a switching element for the image element 104. When it is desired to capture image data from image element 104, control signals 114 are sent to gate driver 112 to xe2x80x9cselectxe2x80x9d the gate of transistor 108. Electrical signals from the photodiode 106 are passed through line 116 to a charge amplifier 110. The output of charge amplifier 110 is sent to a xe2x80x9csample and holdxe2x80x9d stage for further image processing/display. While FIG. 1 only shows four image elements, it is likely that the typical flat panel imager includes many such image elements depending upon the size and resolution of the imager device.
Many imagers perform simultaneous sampling of image data from multiple image elements in a correlated manner. For example, the imager of FIG. 1 collects image data from an entire row or line of image elements at the same time. To form an entire image frame, each row of image data is collected on a row-by-row basis until all rows for the image has been sampled. To obtain image data for a row of image elements, all the switching transistors for image elements on the same row are tied to the same control line extending from gate driver 112. When the image data for a particular row of image elements is desired, control signals 114 are sent to the gate driver 112 to select the transistor gates for the desired row of image elements. The electrical signals from the entire row of image elements are passed to their corresponding charge amplifiers, which outputs signal data to the subsequent sampling stage.
The photodiodes of FIG. 1 are connected to a common node 122 to supply a reverse bias voltage for the image element array. The gate driver 12 is connected to a node 124 to supply low gate voltage to drive the gate control lines. Parasitic capacitance may exist in the imager, such as Cgd to the gate control line and Cad to the common array bias line for each image element. Each amplifier may gain AC noise present on the low gate voltage and array bias voltage depending upon the ratio of the capacitances. If an entire row of image data is sampled at the same time, then the same noise offset may exist for every pixel in that row, which causes the corresponding row of pixels in the final image to appear markedly different from other rows of pixels. This type of xe2x80x9cimage artifactxe2x80x9d is created in the example of FIG. 1 because the low gate voltage and the array bias voltage are common for all amplifiers for a row of data.
FIG. 2a shows an example x-ray image 200 captured using an x-ray imager device. FIG. 2b illustrates the x-ray image 200 of FIG. 1 having examples of image artifacts located thereon. The image artifact 204 is an example of line noise that causes pixel intensity in the affected row of pixels to be lower than pixel intensities for other rows of the image. Thus, the row of pixels corresponding to image artifact 204 appears relatively darker than other image rows. Image artifacts 206 are examples of line noise that cause pixel intensity in the affected rows to be higher than for other rows of the image. Thus, the rows of pixels corresponding to image artifacts 206 appear relatively brighter than other image rows. Compared to random pixel noise, this correlated line noise is often relatively more visible and may significantly degrade image quality.
The present invention is directed to a method and system for reducing correlated noise. According to an aspect of the invention, the invention reduces correlated noise in data generated by imagers by determining a pixel correction value that can be used to adjust image data before begin displayed. In an embodiment of the invention, each set of correlated pixels in a given image frame can be examined to provide a composite image value that can be compared to one or more composite image values for the same set of correlated pixels from prior frame(s). A single composite image value can be generated that represents composite image values for that set of correlated pixels for prior image frames. The composite image value for the present frame is compared to the composite image value for prior frames to determine whether pixel correction is required to reduce correlated noise in the final image. If pixel correction is required, then the set of correlated pixels is adjusted prior to being displayed. Further details of aspects, objects, and advantages of the invention are described below in the detailed description, drawings, and claims.